The present invention relates to a control system for an automatic transmission, and more particularly to a control system for an automatic transmission which is designed to effect shock-free shift between gear ratios.
Laying-open Japanese patent application No. 52-106064 (corresponding to U.S. Pat. No. 4,031,782 issued to Miller et al. on June 28, 1977) discloses a control system for controlling a gear ratio of a gear train. This known control system comprises a control device for controlling the gear ratio of the gear train, and a torque sensor operatively associated with an output member of the gear train to generate an output torque indicative signal. The output torque indicative signal is supplied to the control device such that the gear ratio is controlled in response to the output torque indicative signal. This control system is directed to a precision control of a hydraulic fluid pressure supplied to a friction element which is to be engaged in effecting shifting. Laying-open Japanese patent application No. 53-85264 (corresponding to U.S. Pat. No. 4,102,222 issued to Miller et al. on July 25, 1978) discloses an electronic control system which is directed not only to a precision control of hydraulic fluid pressure supplied to a friction element which is to be engaged, but also to a precision control of hydraulic fluid pressure supplied to another friction element which is to be released. According to this known control system, a torque sensor detects an output torque of a transmission and generates an output torque indicative signal. A first control valve regulates a hydraulic fluid pressure supplied to the friction element which is to be engaged, and a second control valve regulates a hydraulic fluid pressure supplied to the another friction element which is to be released during a shift operation between gear ratios. An error signal is provided to control the operation of each of the first and second control valves. A closed loop controller is operatively coupled with the torque sensor and also with both of the first and second control valves and it receives the output torque indicative signal and generates signals as a function of the output torque indicative signal for controlling the first and second control valves.
In each of the known control systems mentioned above, a torque sensor is operatively associated with an output member of a transmission to detect an output torque and generates an output torque indicative signal. The output torque indicative signal is fed to a controller which computes an error from a desired value in output torque and generates an output signal for controlling a control valve connected to a friction element such that the output torque varies in a predetermined desired pattern, thus alleviating or avoiding substantial shift shocks.
If the feedback control employed by the above mentioned control systems is to be carried out by a control system using a microcomputer, a microcomputer capable of high speed operation is needed. Such a microcomputer is relatively expensive.
If the output signal of the torque sensor is interferred with error components, such as noise, this causes a drop in control precision of the whole system immediately since this is a realtime control. Thus, in order to prevent such a drop in control precision, a relatively expensive torque sensor that provides noise free performance is required.
The present invention aims at solving the above mentioned problems encountered in the known control systems.
An object of the present invention, therefore, is to provide a control system for an automatic transmission which does not require a transducer having a high precision performance nor an arithmetical unit capable of high speed operation.